The Nintendo 3DS slide pad seems to be a large chunk on plastic for just a additional analogue stick and rather than providing additional battery power to your 3DS. The Nintendo 3DS slide pad will require a AAA battery to function.

When it arrives in Japan on December 10th 2011 the Nintendo 3DS slide pad will be priced at ¥1,500 around $19. But unfortunately at the current time no European or US pricing or release date has been announced by Nintendo.